1. Field of the Invention
The present invention relates to a knock control device for an internal combustion engine and, more particularly, to a device for controlling engine knock based on an ion current inside a combustion chamber.
2. Description of the Related Art
In a gasoline engine, the air/fuel mixture in the vicinity of a spark plug is ignited by the spark produced at the spark plug, and gasoline combustion takes place with the ignited flame propagating throughout the entire air/fuel mixture. One abnormal combustion phenomenon that can occur at this time is knocking. Knocking is a condition in which unburned gases self-ignite before the flame front arrives, due to an abnormally rapid rise in pressure during the flame propagation. When knock occurs, combustion gases oscillate, allowing heat to propagate more freely, and in some cases, engine damage may result. Knocking is closely related to ignition timing; as the ignition timing is advanced, maximum combustion pressure increases, increasing the tendency to knock.
On the other hand, it is desirable to increase the compression ratio in order to increase thermal efficiency and reduce fuel consumption. To achieve this, knock control is performed as part of ignition timing control by advancing the ignition timing up to the point where knock is about to occur while detecting the occurrence of knock. Previously, in this kind of knock detection method, it was common practice to detect knock-induced vibrations using a vibration sensor attached to the cylinder block or a like part, but in recent years, a knock detection method has been proposed that utilizes the change that occurs in an ion current inside a cylinder when knock occurs.
More specifically, when a spark is produced at the spark plug and the air/fuel mixture burns in the combustion chamber, the air/fuel mixture is ionized. When a voltage is applied to the spark plug while the mixture is in the ionized state, an ion current flows. The occurrence of knock can be detected by detecting and analyzing this ion current. Usually, when knock occurs, an oscillating component of 6 kHz to 7 kHz appears in the ion current. The knock detection device based on the ion current extracts this frequency component peculiar to knock by means of a filter, and judges the knocking condition based on the magnitude of that component.
As an example, Japanese Unexamined Patent Publication No. 7-286552 discloses a device in which a capacitor as an ion current generating source is charged to a given voltage by the secondary current that flows when the primary current in the ignition coil is shut off, and an ion current that flows, after a spark discharge, through a closed circuit consisting of the capacitor, the secondary winding of the ignition coil, the spark plug and a current detecting resistor, is measured. In such a device, since the secondary winding of the ignition coil (the secondary coil) is located in the ion current flow path, currents other than the ion current also flow.
More specifically, after the end of the discharge at the spark plug, the ignition coil retains residual magnetic energy, and the ignition coil attempts to release this energy, causing LC resonance by interaction between the inductance L of the coil and the stray capacitance C on the high-voltage line. In this way, after the end of the discharge, an LC resonance current due to the coil's residual magnetic energy flows; since this current has no relevance to the ion current, the LC resonance current causes noise (hereinafter referred to as the residual magnetic noise) in knock detection. To address this problem, the above prior art proposes a method in which an output of an ion current detection circuit is masked starting from prescribed timing before the ignition timing, the residual magnetic noise occurring after the end of the spark discharge is directly detected, and the mask is removed when a prescribed time has elapsed from the time the noise was detected, so that only the ion current can be detected after completely removing the residual magnetic noise.
Here, when the ion current output after masking is supplied to a band-pass filter and the band-pass filter output is supplied to an integrator circuit or a peak-hold circuit to extract the frequency component relating to knock oscillations, the period of the integration or peak holding, that is, the gate period, must be set. Usually, this gate period is set in such a manner as to coincide with the period in which knock-induced oscillations appear. The period in which knock-induced oscillations appear depends on the crankshaft angular position and corresponds, for example, to the position from 15.degree. to 60.degree. CA ATDC (crankshaft angle after top dead center).
When the mask period is provided to remove currents other than the ion current, that is, the residual magnetic noise, and also the gate period is provided during which to perform knock detection, as described above, the following problem will arise. That is, since the spark duration varies due to variations (manufacturing variations, etc.) among systems including the coils, changes over time, etc., mask end timing also varies. Furthermore, at high engine rpm, the interval between the mask end timing and the knock occurrence start timing (15.degree. CA ATDC) becomes shorter than at low engine rpm. Therefore, there can occur cases where the mask period continues partially into the gate period.
The following problem arises when the mask period overlaps into the gate period. That is, frequency components are generated over a wide frequency range in association with a signal discontinuity occurring at the time of mask removal, and some of these frequency components are passed unattenuated through the band-pass filter and introduced into the gate period. If the period containing the mask removal-induced noise overlaps into the knock detection gate period, as just described, the accuracy of knock detection degrades and shifting occurs in the set value of the knock evaluation reference level.